This invention relates generally to microturbine power generation systems and more particularly to a microturbine construction and method for providing improved stability and heat transfer through the microturbine engine core.
Microturbines are multi-fuel, modular distributed power generation units having multiple applications. They offer the capability to produce electricity at a lower cost per kilowatt than do central plants, and they do not require the installation of expensive infrastructure to deliver power to the end users. Thus, in parts of the world lacking the transmission and distribution lines of a basic electric infrastructure, commercialization of microturbines may be greatly expedited. In the United States and other countries already having a suitable electric infrastructure, distributed generation units will allow consumers of electricity to choose the most cost-effective method of electric service. In addition to primary power generation, microturbines also offer an efficient way to supply back-up power or uninterruptible power. Other applications for microturbines exist as well.
Structurally, engine cores of present-day microturbine power generating systems include a compressor, a turbine for converting gaseous heat energy into mechanical energy, and an electrical generator for converting the mechanical energy produced by the turbine into electrical energy. The electrical generator includes a rotor and a stator. The rotor is mechanically coupled to wheels of the turbine and the compressor. While some proposed designs for microturbines include oil-lubricated ball bearings, microturbines can advantageously incorporate gas bearings instead. As used herein, "air bearings" are a subset of gas bearings--for example, gas bearings in which the operating medium is air obtained from the environment surrounding the microturbine.
If gas bearings are used in a microturbine, the above-described combination of rotor, compressor and turbine are rotatably supported by the gas bearings. The gas bearings in a common configuration include fluid film journal and thrust bearings. A microturbine engine core that uses gas bearings includes a single moving part, which allows for low technical skill maintenance and a high level of reliability.
Because unwanted heat can be generated by the engine core of a microturbine power generating system, it is desirable to include design features that allow for cooling of the electrical generator components, including the stator and the electrical conductor therein (e.g., stator wires). When the stator is of conventional, multi-tooth design, one method for cooling the stator involves passing cooling fluid, such as water or glycol, through a sleeve that surrounds the stator to transfer stator heat to the fluid. The fluid then may be cooled in a heat exchanger and passed back through the cooling sleeve surrounding the stator. Alternatively, a continuous supply of cool water may be used and, after it is heated by the unwanted stator heat, passed outside the microturbine power generating system for other uses. However, while the use of a fluid-cooled conventional stator offers design opportunities, it also presents certain problems, including problems associated with a microturbine that uses gas bearings. Specifically, cooling of air bearings, a rotor, and a stator end turn becomes problematic. Furthermore, stator end turn cooling typically requires special cooling flow components.
In the present microturbine cooling system, however, air bearing cooling flow--which is already required--performs the secondary function of stator end turn cooling. Using the existing cooling flow system for stator end turn cooling results in a simpler, lower cost microturbine.
Additionally, it is well known that air bearing damping and load capacity are a function of their operating pressure. The design arrangement of the present invention operates the air bearings at or near their highest possible pressures, resulting in a significant improvement in rotor dynamic stability because of improved bearing damping and load capacity. The present invention offers several other advantages as well.